1. Field Of the Invention
The subject invention generally pertains to torsion springs of the bar type, such as torsion bar springs used in some automotive suspensions. More specifically, the invention relates to the provision of a stiffener for a torsion bar spring.
2. Description of Related Art
In the design of automotive suspensions it is occasionally preferable to use torsion springs rather than the more widely used coil springs. Typically a vehicle will have two torsion bars, one for each front wheel. The rear wheels generally use leaf or coil springs. These torsion bars are used as part of the suspension system that suspends the body of the vehicle from the wheels. The ride characteristics of a vehicle are determined by the rate of movement between the wheels and vehicle body under various conditions. This separation of wheel movement from body movement is determined by the weight of the vehicle, the weight of the wheel assembly, the stiffness of the springs and the amount of damping from the shock absorbers used to limit the velocity of the wheels relative to the vehicle body. The appropriate torsion bar stiffness is selected based upon normal vehicle operations. If one or more of the vehicle components change such as the vehicle weight, the damping rate of the shock absorbers, or the weight of the wheel assembly, such as with heavier or lighter wheels and tires, or addition of a stabilizer bar, a corresponding change in spring rate is desirable to maintain or improve the vehicles ride characteristics. When torsion bars are designed as part of a vehicles suspension system, the torsion bars are increased in diameter when a higher spring rate is desired.
Many of the changes made by the vehicle owner can affect the ride characteristics. Larger diameter torsion bars can be purchased to replace original torsion bars. This is seldom done by the vehicle owner and is generally delegated to experienced mechanics, and so becomes an expensive proposition. As a result, many vehicle owners do not change torsion bars even though the ride characteristics may suffer dramatically. Having to replace torsion bars to increase the stiffness is a major obstacle.
The majority of prior work has related specifically to stabilizer bars on vehicles. Stabilizer bars are a single torsion spring that attaches the front or rear wheels transversely through bearing attachments to the vehicle chassis. Stabilizer bars effect the roll stiffness of the vehicle during cornering. The suspension stiffness remains unaffected except during cornering and during rough road conditions. Devices thus far used to increase the stiffness of torsion bar springs are limited to the design of original torsion bars and in particular stabilizer bars.
U.S. Pat. No. 4,648,620, is a concept that modifies the spring rate of a stabilizer bar when activated providing two possible spring rates. This secondary torsional reaction element must be welded onto the primary torsional reaction unit in order for the torsion to be transferred to the secondary torsional reaction unit when engaged. Welding is detrimental to torsion bars used as vehicle springs, except when properly heat treated as an assembly.
U.S. Pat. No. 4,884,790 describes a nonlinear torsion bar spring that increases the stiffness as a function of the angle of twist. The primary torsion reaction unit requires a longitudinal machined extrusion or depression be part of the engagement section that transfers load to the secondary torsion reaction unit. This machining as well as attachment of the secondary unit permanently to the primary torsion reaction unit precludes the use of the original torsion bar. The means of attachment cannot be by frictional means due to the length of the secondary torsion reaction unit and the reaction loads that would be applied. This bar also as described requires removal from the vehicle to be installed.